The present invention concerns a process for making a copper connection through a layer of dielectric material in an integrated circuit.
The improvements in the performance of integrated circuits (speed, low consumption) have required a change in some of the materials that have been used up till now. In order to reduce the capacity that exists between the conductive lines formed in one or several layers of dielectric material (damascene-type structures), it is advantageous to use, as the dielectric material, materials with a very low dielectric constant. These materials, called xe2x80x9clow-kxe2x80x9d materials, comprise organic materials and porous materials.
The improvements in the performance have also been achieved through the use of a metal that is more conductive than aluminium, which has traditionally been used to make the interconnection lines. Copper, whose resistivity is nearly half that of aluminium doped with copper, has shown itself to be the best candidate.
FIGS. 1 and 2 show the preparatory steps for making a cross over (or via hole) in the copper in the case of a single damascene structure, according to the prior art.
FIG. 1 shows a layer of dielectric material 1 covering a semiconductor substrate (not shown), with this layer comprising a connection element 2 made out of copper that is level with the surface of the dielectric material 1. An encapsulation layer 3 is deposited on the surface of the layer 1. A layer of dielectric material 4 is deposited on the encapsulation layer 3. A hard masking layer 5 is deposited on the layer 4 in order to define the location of the via hole 6, obtained by etching. The encapsulation layer 3 may be made out of SiN. It acts as a protective layer for the layer of dielectric material 4 by preventing the contamination of this dielectric material by the copper in the connection element 2. It also acts as a barrier layer for the etching of the layer 4.
As shown in FIG. 2, the encapsulation layer 3 is then etched in the bottom of the hole 6, for example by a photolithographic process, in order to reveal the copper connection element 2 that needs to be contacted.
The following steps comprise spraying a layer of copper onto the hard masking layer 5 while filling the hole 6 in order to make contact with the connection element 2. Chemical-mechanical polishing of the copper until the level of the hard mask is reached makes it possible to eliminate the excess copper and obtain the via hole in the copper.
After etching the encapsulation layer 3, two harmful effects can occur. The first harmful effect of this etching consists in spraying the copper onto the walls of the hole 6 from the revealed surface of the connection element 2. The second harmful effect consists in a pollution of the surface of the connection element 2 as a function of the type of chemistry used in the etching process.
In order to both eliminate the copper from the walls of the via holes and restore the condition of the surface of the copper at the bottom of the holes, those skilled in the prior art use dry and wet chemical methods. For example, when the layers of dielectric material are made out of SiO2 or SiN, a cleaning sequence containing diluted HF and using a reactive plasma containing hydrogen is efficient.
When the dielectric material of the damascene structure is a xe2x80x9clow-kxe2x80x9d material, several problems arise. As the copper is sprayed after etching the encapsulation layer, it may cause in-depth contamination of the layer of xe2x80x9clow-kxe2x80x9d material (in the case of a polymer or a porous material). There is then a deterioration in the local dielectric properties (for example, the dielectric constant, the discharge field and the leakage current) of this material. The cleaning processes may not be able to be used because of the risk of attack, or even deterioration, of the dielectric material that these processes may cause. Moreover, these cleaning processes do not allow decontamination of the bulk of dielectric material. In the case of a xe2x80x9clow-kxe2x80x9d material such as SiLK, the use of hydrogen fluoride is advised against because it diffuses into this material and causes the hard mask to disbond. At this moment in time, there is no cleaning solution for SiLK. Those skilled in the art therefore do their best to avoid its contamination.
In addition, the use of xe2x80x9clow-kxe2x80x9d material generally causes problems in the mechanical resistance of the structures. When a polymer based dielectric material or a porous dielectric material is used, offsets can occur in the via holes due to the fact that the polymer is a relatively soft material or, in the case of a porous dielectric material, during the chemical-mechanical polishing of the copper.
The invention provides a solution to the problems described above.
The object of the invention is a process for making a copper connection with a copper connection element in an integrated circuit comprising a damascene structure, with the connection element being covered successively with an encapsulation layer and at least one layer of dielectric material with a very low dielectric constant (called a xe2x80x9clow-kxe2x80x9d material), the process comprising the following steps:
etching said layer of dielectric material until the encapsulation layer is reached in order to obtain a connection hole, opposite the connection element,
forming a protective layer on the walls of the connection hole, with the protective layer enabling contamination of the layer of dielectric material by diffusion of copper to be avoided,
etching the encapsulation layer, at the bottom of the connection hole, in such a way as to reveal the connection element,
filling the connection hole with copper.
The protective layer that remains on the walls of the hole prevents contamination of the dielectric material. It makes it possible for the connection hole to be cleaned without any risk. It reinforces the mechanical rigidity of the structure. It is also advantageous if spacers are used, providing they do not act as a barrier to the diffusion of the copper.
After the anisotropic etching of the encapsulation layer and before the step of filling the connection hole, the process may comprise a step of cleaning the walls and the bottom of the connection hole. If the protective layer is made out of a material that acts as a barrier to the diffusion of copper, this cleaning step of the walls of the hole can be omitted.
The encapsulation layer may be a layer of SiN.
The layer of dielectric material with very a low dielectric constant may be a material chosen among polymers and porous materials.
If the layer of dielectric material is covered with a hard masking layer, the process may comprise a preliminary step of forming, in the hard masking layer, an opening opposite the connection element, with the etching of the layer of dielectric material being achieved through the opening in the hard masking layer. The hard masking layer may be a layer of material chosen from silicon oxide, silicon carbide and silicon nitride.
The step of depositing a protective layer may consist in depositing a layer of metallic material, such as SiO2, SiCH, TiN or SiN.